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Data and Computer Communications

Data and Computer Communications. Transmission Media . Transmission Media.

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Data and Computer Communications

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  1. Data and Computer Communications Transmission Media

  2. Transmission Media Communication channels in the animal world include touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. —Mind of the Raven, Bernd Heinrich

  3. Overview • transmission medium is the physical path between transmitter and receiver • guided media – guided along a solid medium • unguided media – atmosphere, space, water • characteristics and quality determined by medium and signal • guided media - medium is more important • unguided media - bandwidth produced by the antenna is more important • key concerns are data rate and distance

  4. Design Factors Determining Data Rate and Distance

  5. Electromagnetic Spectrum

  6. Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Transmission Characteristics of Guided Media

  7. Guided Transmission Media

  8. Twisted Pair • Twisted pair is the least expensive and most widely used guided transmission medium. • consists of two insulated copper wires arranged in a regular spiral pattern • a wire pair acts as a single communication link • pairs are bundled together into a cable • most commonly used in the telephone network and for communications • within buildings

  9. Twisted Pair - Transmission Characteristics susceptible to interference and noise

  10. Unshielded vs. Shielded Twisted Pair

  11. Twisted Pair Categories and Classes

  12. Near End Crosstalk • coupling of signal from one pair of conductors to another • occurs when transmit signal entering the link couples back to the receiving pair - (near transmitted signal is picked up by near receiving pair)

  13. Signal Power Relationships

  14. Coaxial Cable • Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair. • consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor • is a versatile transmission medium used in a wide variety of applications • used for TV distribution, long distance telephone transmission and LANs

  15. Coaxial Cable - Transmission Characteristics

  16. Optical Fiber • Optical fiber is a thin flexible medium capable of guiding an optical ray. • various glasses and plastics can be used to make optical fibers • has a cylindrical shape with three sections – core, cladding, jacket • widely used in long distance telecommunications • performance, price and advantages have made it popular to use

  17. Optical Fiber - Benefits • greater capacity • data rates of hundreds of Gbps • smaller size and lighter weight • considerably thinner than coaxial or twisted pair cable • reduces structural support requirements • lower attenuation • electromagnetic isolation • not vulnerable to interference, impulse noise, or crosstalk • high degree of security from eavesdropping • greater repeater spacing • lower cost and fewer sources of error

  18. Optical Fiber - Transmission Characteristics • uses total internal reflection to transmit light • effectively acts as wave guide for 1014 to 1015 Hz (this covers portions of infrared & visible spectra) • light sources used: • Light Emitting Diode (LED) • cheaper, operates over a greater temperature range, lasts longer • Injection Laser Diode (ILD) • more efficient, has greater data rates • has a relationship among wavelength, type of transmission and achievable data rate

  19. Optical Fiber Transmission Modes

  20. Frequency Utilization for Fiber Applications WDM = wavelength division multiplexing

  21. Attenuation in Guided Media

  22. Wireless Transmission Frequencies

  23. Antennas • electrical conductors used to radiate or collect electromagnetic energy • same antenna is often used for both purposes

  24. Radiation Pattern • power radiated in all directions • does not perform equally well in all directions • as seen in a radiation pattern diagram • an isotropic antenna is a point in space that radiates power • in all directions equally • with a spherical radiation pattern

  25. Parabolic Reflective Antenna

  26. Antenna Gain • measure of the directionality of an antenna • power output in particular direction verses that produced by an isotropic antenna • measured in decibels (dB) • results in loss in power in another direction • effective area relates to physical size and shape

  27. Terrestrial Microwave

  28. Terrestrial Microwave Applications used for long haul telecommunications, short point-to-point links between buildings and cellular systems used for both voice and TV transmission fewer repeaters but requires line of sight transmission 1-40GHz frequencies, with higher frequencies having higher data rates main source of loss is attenuation caused mostly by distance, rainfall and interference

  29. Microwave Bandwidth and Data Rates

  30. Satellite Microwave • a communication satellite is in effect a microwave relay station • used to link two or more ground stations • receives on one frequency, amplifies or repeats signal and transmits on another frequency • frequency bands are called transponder channels • requires geo-stationary orbit • rotation match occurs at a height of 35,863km at the equator • need to be spaced at least 3° - 4° apart to avoid interfering with each other • spacing limits the number of possible satellites

  31. Satellite Point-to-Point Link

  32. Satellite Broadcast Link

  33. Satellite Microwave Applications • uses: private business networks • satellite providers can divide capacity into channels to lease to individual business users television distribution • programs are transmitted to the satellite then broadcast down to a number of stations which then distributes the programs to individual viewers • Direct Broadcast Satellite (DBS) transmits video signals directly to the home user global positioning • Navstar Global Positioning System (GPS)

  34. Transmission Characteristics • the optimum frequency range for satellite transmission is 1 to 10 GHz • lower has significant noise from natural sources • higher is attenuated by atmospheric absorption and precipitation • satellites use a frequency bandwidth range of 5.925 to 6.425 GHz from earth to satellite (uplink) and a range of 3.7 to 4.2 GHz from satellite to earth (downlink) • this is referred to as the 4/6-GHz band • because of saturation the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GH

  35. Broadcast Radio • radio is the term used to encompass frequencies in the range of 3kHz to 300GHz • broadcast radio (30MHz - 1GHz) covers • FM radio • UHF and VHF television • data networking applications • omnidirectional • limited to line of sight • suffers from multipath interference • reflections from land, water, man-made objects

  36. Infrared • achieved using transceivers that modulate noncoherent infrared light • transceivers must be within line of sight of each other directly or via reflection • does not penetrate walls • no licenses required • no frequency allocation issues • typical uses: • TV remote control

  37. Frequency Bands

  38. Wireless PropagationGround Wave • ground wave propagation follows the contour of the earth • and can propagate distances well over the visible horizon • this effect is found in frequencies up to 2MHz • the best known example of ground wave communication • is AM radio

  39. Wireless PropagationSky Wave • sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America • a signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere back down to earth • sky wave signals can travel through a number of hops, bouncing back and for the between the ionosphere and the earth’s surface

  40. Wireless PropagationLine of Sight • ground and sky wave propagation modes do not operate above 30 MHz - - communication must be by line of sight

  41. Refraction • velocity of electromagnetic wave is a function of the density of the medium through which it travels • ~3 x 108 m/s in vacuum, less in anything else • speed changes with movement between media • index of refraction (refractive index) is • sine(incidence)/sine(refraction) • varies with wavelength • gradual bending • density of atmosphere decreases with height, resulting in bending of radio waves towards earth

  42. Line of Sight Transmission

  43. Free Space Loss

  44. Multipath Interference

  45. Summary • transmission Media • physical path between transmitter and receiver • bandwidth, transmission impairments, interference, number of receivers • guided Media • twisted pair, coaxial cable, optical fiber • wireless Transmission • microwave frequencies • antennas, terrestrial microwave, satellite microwave, broadcast radio • wireless Propagation • ground wave, sky wave, line of sight

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